I don't have the slim whistle. There have been reports on other fora of the plastic insert moving inappropriately. 4Sevens reports that they have found the problem in production and fixed it. They also say that they will fix or replace any that have the issue.

My keychain whistle is the flat $40 model. No problems to report. Would buy again without hesitation.

I have no affiliation with 4Sevens other than owning a ridiculous amount of their products.